Linear alpha-olefins are important materials used as comonomers, cleaners, lubricants, plasticizers, etc. and are commercially and widely used. Particularly, 1-hexene and 1-octene are widely used as comonomers for controlling the density of polyethylene during the preparation of linear low-density polyethylene (LLDPE).
In a preparation process of common LLDPE, copolymerization of alpha-olefins, for example, 1-hexene, 1-octene, etc. with ethylene is performed to control density by forming branches on a polymer backbone.
Accordingly, in the preparation of LLDPE having a high comonomer content, the comonomer is a costly part. To solve the drawback, various methods have been conducted.
In addition, the application field or the market size of alpha-olefins is dependent on the kind thereof, and technique on selective production of a specific olefin is commercially very important. Recently, research on a technique using a chromium catalyst for preparing 1-hexene or 1-octene with high selectivity via selective ethylene oligomerization is being actively conducted.
Conventional and commercial preparation methods of 1-hexene or 1-octene include the Shell higher olefin process (SHOP) of Shell Chemicals, the Ziegler process of Chevron Philips Chemical, etc. Through the methods, alpha-olefins having a wide distribution of C4-C20 may be obtained.
As a catalyst for trimerizing ethylene, a chromium-based catalyst using a ligand having a formula of (R1)(R2)X—Y—X(R3)(R4) is suggested. In the formula, X is phosphorus, arsenic or antimony, Y is a linking group such as —N(R5)-, and at least one of R1, R2, R3 and R4 have a polar or electron donating substituent.
In addition, a compound of (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2 comprising no polar substituent for at least one of R1, R2, R3 and R4 has been studied as a ligand not exhibiting catalyst activity to 1-hexene under catalytic conditions (Chem. Commun., 2002, 858).
However, the conventional ligand comprising a heteroatom is still required to have consistent and continuous activity on multimerization reaction and high selectivity during preparing 1-octene or 1-hexene.